Added partial days support to `DateTime`'s `advance` method.

You can now add partial days (e.g. 2.5.days) to `DateTime` with the advance method.
This was acheived by mimicing the `advance` implementation in `Time`.
Fixed changelog
WIP: pass existing joins to construct_join_dependency
Merge pull request #15210 from arthurnn/fix_hbtm_reflection

Fix habtm reflection
Conflicts:
    activerecord/CHANGELOG.md
    activerecord/lib/active_record/counter_cache.rb
    activerecord/lib/active_record/reflection.rb
    activerecord/test/cases/reflection_test.rb
Use .to_s on _reflections lookup
Skip if the association was already loaded.
Call get_routes_as_head only on HEAD requests.
Rename Posts to Articles in Guides' Getting Started App, continuation of #15215 [ci skip]
remove unwanted `to_sym` call.
update the recommended ruby version
Merge pull request #15333 from ShunsukeAida/update_ruby_version

update the recommended ruby version.
correct doc for (audio|video)_tag [ci skip]
Escape user input before showing in the page.

This is not a security issue since this page is not present in
production and user have to type something in the field but is better to
escape the input.
Skip individual tests upon Dalli::RingError

Unlike the outer `rescue`, this one is much more precise about what we
want to handle: a connection failure (`Dalli::RingError`) is not
relevant to what we're testing here. But other Dalli errors may well be
indicating an actual problem.
Merge pull request #15342 from aditya-kapoor/correct-tag-docs

correct docs for audio_tag and video_tag [ci skip]
Merge pull request #15345 from matthewd/skip-dalli-errors

Skip individual tests upon Dalli::RingError
correct docs for (asset|image)_path [ci skip]
Merge pull request #15346 from aditya-kapoor/correct-doc-for-path

correct documentation for asset_path and image_path [ci skip]
Merge pull request #15323 from JohnKellyFerguson/getting-started-app

Docs: Rename Posts to Articles in Guides' Getting Started App
Refactor the handling of arrays in where predicates

Simplifies the code slightly, isolates non-nil non-range values into a
single array, which will make it easier to do things like apply type
casting to them in the future.